syscalls/setreuid: 16-bit fixes and minor cleanup

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
syscalls/setregid{01, 03, 04}: 16-bit fixes and minor cleanup

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
syscalls/setregid02: fixes

It was a bug that the testcase initially changed its uid/gid
to 'nobody' user but after performing setregid() It waited for
'users' gid. Fixed this.

Also added 16-bit version check and performed minor cleanup.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>